As a prior art relating to a multiple output DC-DC converter that receives a DC voltage and supplies controlled DC voltages to a plurality of loads, a configuration shown in FIG. 13 is available. The prior-art multiple output DC-DC converter shown in FIG. 13 is a power source for driving a liquid crystal panel installed in a portable apparatus. In this multiple output DC-DC converter, an input DC voltage Ei=2.5 to 3.3 V is supplied from a battery serving as an input power source 1; a first output voltage Vout1=3.5 V is output as a source drive voltage; and a second output voltage Vout2=13.5 V and a third output voltage Vout3=−13.5 V are output as positive and negative gate drive voltages. A first converter 100 is provided with a first main switch 102 formed of an N-channel MOSFET, a first inductor 103, a first diode 105 for output (first output diode 105) and a first capacitor 106 for output (first output capacitor 106). A step-up converter for outputting the first output voltage Vout1 from the first output capacitor 106 to a first load 107 is formed by the first converter 100. A first control circuit 108 adjusts the ON/OFF ratio of the first main switch 102 to control the first output voltage Vout1. A second converter 200 is provided with a second main switch 202 formed of an N-channel MOSFET, a second inductor 203, a second diode 205 for output (second output diode 205) and a second capacitor 206 for output (second output capacitor 206). A step-up converter for outputting the second output voltage Vout2 from the second output capacitor 206 to a second load 207 is formed by this second converter 200. A second control circuit 208 adjusts the ON/OFF ratio of the second main switch 202 to control the second output voltage Vout2. A third converter 300 receives the second output voltage Vout2 and is provided with a first switch 301, a second switch 302, a capacitor 303, a first diode 304, a second diode 305 and a third capacitor 306 for output (third output capacitor 306). An inverting-type switched capacitor for outputting the third output voltage Vout3 from the third output capacitor 306 to a third load 307 is formed by this third converter 300. A third control circuit 308 carries out control so as to alternately turn ON and OFF the first switch 301 and the second switch 302.
The operation of the prior-art multiple output DC-DC converter shown in FIG. 13 will be described below briefly. First, in the first converter 100, when the first main switch 102 is ON, the input DC voltage Ei is applied to the first inductor 103. At this time, current flows through the first inductor 103 and magnetic energy is stored. Then, when the first main switch 102 becomes OFF, the magnetic energy stored in the first inductor 103 is released as a current for charging the first output capacitor 106 via the first output diode 105. When it is assumed that the first main switch 102 turns ON/OFF in a constant cycle, the energy being output every cycle via the first inductor 103 becomes larger as the ON period of the first main switch 102 is longer. Accordingly, the first output voltage Vout1 becomes higher as the ON period of the first main switch 102 is longer. In other words, the first control circuit 108 adjusts the ON/OFF period ratio of the first main switch 102, whereby the first output voltage Vout1 is controlled. Similarly, in the second converter 200, the control circuit 208 adjusts the ON/OFF period ratio of the second main switch 202, whereby the magnetic energy stored in the second inductor 203 is released as a current for charging the second output capacitor 206 via the second output diode 205. The release amount of this magnetic energy is adjusted, whereby the second output voltage Vout2 is controlled.
In the third converter 300, when the first switch 301 is ON, the second output voltage Vout charges the capacitor 303 via the second diode 305. When the second switch 302 is ON, the energy of the capacitor 303 charges the third output capacitor 306 via the first diode 304, whereby the third output voltage Vout3, obtained by inverting the second output voltage Vout2 to its negative side, is output.
In the above-mentioned configuration, in order to output three kinds of different voltages, three converters are required. However, in portable apparatuses, the number of components is requested to be made smaller, even a single component, in order to attain miniaturization and weight reduction. As means for controlling a plurality of outputs by using smaller number of components, an art described in Japanese Patent Examined Publication No. Hei 7-40785 is available, for example. FIG. 14 is a circuit diagram of a step-up converter having three outputs disclosed in FIG. 1 of Japanese Patent Examined Publication No. Hei 7-40785. In FIG. 14, magnetic energy from an input DC power source V11 is stored while a switch S1 is in contact with a contact 1. The magnetic energy is released to the output side while the switch S1 is in contact with a contact 2. At that time, the magnetic energy is distributed to each output by a switch S2. The invention disclosed in Japanese Patent Examined Publication No. Hei 7-40785 indicates a method for stabilizing the output of each output by controlling the ON period in which the switch S2 is in contact with each contact and for controlling the switch S1 so that electricity is supplied to all loads just sufficiently.
Japanese Patent Examined Publication No. Hei 7-40785 discloses a system wherein the contacts of the switch S2 are switched by time-sharing the period in which the switch S1 is in contact with the contact 2 (the OFF period of the main switch). Although being different in circuit configuration from this prior art, an invention relating to a different control method configured on the basis of a similar technical concept is known. For example, the specification of U.S. Pat. No. 5,400,239 discloses an insulation-type flyback converter having N outputs. In this insulation-type flyback converter, N rectifying and smoothing circuits are connected to one output winding of a transformer via a switch corresponding to the switch S2. Furthermore, the switching frequency of the main switch is divided by N and assigned to the control of each output. In other words, when this insulation-type flyback converter is replaced with the configuration shown in FIG. 14, the switch S2 is switched in accordance with a 1/N switching frequency, and the ON period of the switch S1 is adjusted every switching cycle, whereby each output voltage is controlled.
In addition, the specification of U.S. Pat. No. 5,751,139 discloses a multiple output non-insulation-type DC-DC converter provided with one inductor. In this non-insulation-type DC-DC converter, priority is given to outputs desired to be stabilized. When the configuration of this non-insulation-type DC-DC converter is replaced with the configuration shown in FIG. 14, the switch S2 selects outputs in accordance with order of priority and supplies electricity; when the voltage of a selected output reaches its upper limit threshold value, switching is carried out to select the output having the next order of priority.
As described above, in the prior-art apparatuses, the output of a single DC-DC converter is time-shared so as to be supplied to a plurality of outputs, and a main switch and an output selection switch are controlled to stabilize each output. When this kind of technology is applied to the prior-art multiple output converter shown in FIG. 13, it is necessary to use an output selection switch corresponding to the switch S2 (FIG. 14). However, in the first converter 100 and the second converter 200, the first main switch 102 can be used as the second main switch 202, and the first inductor 103 can be used as the second inductor 203, whereby one inductor, being large in volume among the components, can be eliminated.
As described above, in order to control the three outputs, that is, the first to third output voltages, the prior-art multiple output DC-DC converter comprises two step-up converters and one inverting-type switched capacitor. However, in portable apparatuses desired for miniaturization and weight reduction, in particular, the number of components is requested to be made smaller. In the prior-art technology, in order to control the plurality of outputs by using smaller number of components, it is possible to reduce one inductor by integrating the two step-up converters into one. However, in order to generate a negative output voltage, the inverting-type switched capacitor shown in FIG. 13 is required. It is difficult to finely adjust the output voltage by using this inverting-type switched capacitor. Furthermore, since the capacitor charging and discharging currents are surge currents, problems due to switching noise and switching loss are caused. It is considered to install an inverting converter as a method for obtaining any given negative output voltage; however, an inductor becomes necessary instead of the capacitor used for the inverting-type switched capacitor. Since this kind of inductor is large in volume among the components of the converter, apparatus miniaturization and weight reduction are obstructed.
The present invention is intended to provide a single DC-DC converter capable of outputting a plurality of voltages stepped up from an input voltage and having the same polarity as that of the input voltage or having the opposite polarity, in other words, a multiple output DC-DC converter capable of controlling a plurality of outputs by using one inductor. The present invention provides a multiple output DC-DC converter capable of reducing the number of components and miniaturizing the entire size of the circuit.